Quinolonecarboxylic acid derivatives are widely used in medicine as synthetic antibacterials. However, emergence of resistant bacteria represented by MRSA has become a major obstacle in such treatments. A quinolonecarboxylic acid derivative represented by the following formula (A):
is a compound capable of overcoming the problems associated with various resistant bacteria since this not only exhibits excellent effects on MRSA but also has an antibacterial activity against resistant Gram-positive bacteria. A known process for producing an intermediate having the mother nucleus of quinolonecarboxylic acid is shown by the following reaction scheme (see, for example, Patent Document 1).

Patent Document 1: WO 02/040478 gazette